Liquid dispensing systems have become an integral part of the electronics manufacturing process for depositing underfill, encapsulants, solder fluxes, surface mount adhesives, conformal coatings and other materials onto a substrate, such as a printed circuit board. Each liquid dispensing system used in the electronics manufacturing process has a particular dispensing characteristic that is determined in large measure by the desired liquid dispense pattern on the substrate, the liquid flow rate and/or liquid viscosity of the dispensed material, and the desired electronic component assembly throughput through the dispensing system.
In the high speed dispensing, or pumping, of precise amounts of liquid for deposition onto a substrate, such as solder paste, it is common to use an auger, or screw, dispenser. Use of an auger dispenser helps achieve a high degree of repeatability and control of the liquid being dispensed. These features are important because of the high speed nature of this deposition process. For instance, in depositing “droplets” of solder paste onto a substrate, it is necessary in some applications to dispense the liquid at a rate of 25,000 dots per hour, or just less than 7 dots per second. Under these conditions, a high degree of liquid control is necessary in order to achieve sufficient repeatability and accuracy in the deposition process.
In an auger dispenser, an auger is held within a dispensing head. The auger extends along an axial flowpath for the liquid to be dispensed, with an outlet end of the auger located adjacent an exit port of the flowpath, and an opposite, drive end of the auger held by and rotatably driven by a coupling, which operatively connects to a drive motor via a drive shaft. The exit port of the flowpath is located at the terminal end of a dispensing tip that extends from the auger dispenser toward the substrate. The dispensing head has an inlet in fluid communication with the flowpath, and a liquid supply line supplies dispensing liquid to the flowpath through the inlet. Initially, liquid is supplied to the flowpath under pressure, to fill the flowpath along the length of the auger, and then the pressure is reduced to a lower operating level. Thereafter, incremental rotations of the drive motor cause rotation of the auger, thereby producing “droplets” of liquid, such as solder paste, to flow from the dispensing tip for deposition onto a substrate which is usually clamped in a fixed position.
In some instances, angular rotation of the motor drive shaft of about 1/16 of a full rotation results in a “droplet” of liquid exiting from the dispensing tip. Typically the dispenser is moved relative to the substrate, and discrete successive partial rotations of the motor shaft, and the auger coupled thereto, results in repeated deposition of liquid onto the substrate.
In “floating head” deposition of liquid onto a substrate, the dispensing head is operatively connected to a robotic control mechanism that moves the dispensing head in a vertical direction toward the substrate until a standoff, carried by the dispensing head, contacts the substrate. The dispensing head is mounted to “float” on the robotic control mechanism so that some amount of “play” is built into the system to accommodate the repeated impact of the dispensing head with the substrate. The standoff extends toward the substrate and has a length slightly greater than the length of the dispensing tip. In this way, very accurate and repeatable spacing of the dispensing tip away from the substrate is achieved when the standoff is positioned to contact the substrate. Liquid dispensing occurs when the standoff contacts the substrate, which occurs repeatedly during the liquid deposition process. The dispensing head moves toward the substrate until the standoff contacts the substrate, a liquid “droplet” is deposited on the substrate, and then the dispensing head moves away from the substrate. The dispensing head is then repositioned relative to the substrate and the liquid deposition cycle repeats again.
Prior to a dispensing cycle, the robotic control mechanism must be programmed to accurately move the dispensing head from a position spaced from the substrate to a position at which the standoff contacts the substrate to properly space the dispensing tip away from the substrate for a liquid dispensing cycle. The position of the dispensing head must be adjusted in the set-up procedure to ensure that the standoff contacts, but does not place significant force upon, the substrate during each dispensing cycle.
In the past, the initial set-up of the dispensing head position has been performed manually by an operator who adjusts and sets the dispense position of the dispensing head relative to the substrate by eye. For example, the operator moves the dispensing head toward the substrate until the standoff contacts the substrate. The robot control mechanism is instructed to “learn” the stroke or travel distance the dispensing head has moved to reach this position, and this value is then stored in memory as a “nominal” stroke or travel distance. For each dispensing cycle to follow, the dispensing head is programmed to move slightly beyond the stored “nominal” stroke or travel distance toward the substrate to ensure that the standoff contacts the substrate even when the substrate is warped or bowed in a direction beyond the “nominal” stroke.
Alternatively, the dispensing head may include a mechanical or optical probe mechanism, in addition to the standoff, that is capable of sensing contact of the dispensing head with the substrate. During initial set-up using the probe mechanism, the dispensing head is moved toward the substrate until the probe mechanism senses contact with the substrate. When this contact is sensed, the robot control mechanism is instructed to store the stroke or travel distance the dispensing head has moved as a “nominal” stroke or travel distance. The dispensing head is programmed to move slightly beyond the stored “nominal” stroke or travel distance toward the substrate to ensure proper positioning of the dispensing head for most conditions of the substrate as described above.
Regardless of which initial set-up procedure is used, it is possible that the programmed dispense position of the dispensing head may change over time due to variations in the positioning accuracy of the robotic control mechanism. These changes can lead to undertravel of the dispensing head toward the substrate wherein the dispensing tip is positioned too far away from the substrate so that the standoff does not contact the substrate. With undertravel, the desired formation of “droplets” onto the substrate is not achieved and the part must be reworked or scrapped. Alternatively, these changes can lead to overtravel of the dispensing head toward the substrate to a position beyond the built-in “play” or “floating” capability of the floating head liquid dispenser. With overtravel, the dispensing head may be moved toward the substrate so far that the dispensing tip actually pushes through the substrate, thereby ruining the part and even possibly damaging the floating head liquid dispenser as well. Undertravel and overtravel conditions can also occur when the substrate is severely warped so that proper contact of the standoff with the substrate is not achieved when the dispensing head is moved through the programmed stroke. In the past, these undertravel and overtravel failure modes of operation have been detectable only when an operator is present to see these failure conditions occur. As a result, significant part waste, and even equipment damage, can occur when the dispensing process is left unattended.
Thus, there is a need for a floating head liquid dispenser that provides simplified, yet accurate, reliable and repeatable position set-up of the dispensing head for a dispensing cycle.
There is also a need for a floating head liquid dispenser that reduces the potential for part waste and equipment damage during unattended dispensing of liquid onto a substrate in a “floating head” dispensing environment.